I propose to study the early development of a small set of vertebrate neurons to learn how their detailed morphologies vary as a function of where and when they develop in the brain. I will study 5 types of hindbrain reticulospinal neurons of the zebrafish that can be specifically identified within 3 days after fertilization by backfilling them with horseradish peroxidase (HRP). The largest and best known of these is the Mauthner neuron. The experiments test predictions of an hypothesis that differences in the axonal and dendritic projections of these neurons can be explained by differences in the times when these neurons undergo steps in their development. I propose to examine this directly by learning the time when these neurons originate (by thymidine labeling and autoradiography), when their axons and dendrites grow out, and when some of their afferents appear (by HRP filling the developing neurons and their afferents). In addition I will examine the role of cell position by correlating changes in morphology with changes in position of experimentally dislocated Mauthner neurons. The investigations will provide basic information about mechansims of neuron patterning in this evolutionarily conservation region of the brain.